Logging while drilling



y 1956 s. A. SCHERBATSKOY 2,755,431

LOGGING WHILE DRILLING Original Filed July 11, 1950 DEPTH METER TERNSDUCBE l 7 J MICIZOPHONE 5 SEQGE Fl. SCHEJZBQTSKOV INVENTOR.

United States Patent LOGGING WHILE DRILLING Serge A. Scherbatskoy, Tulsa, Okla.

Original application July -11, 1950, Serial No. 173,165. and this application June 27, 1952, Serial No.

18 Claims. (Cl. 324-1) This invention relates in general to exploration of geological strata traversed by .earth boreholes, and more particularly to methods and apparatus for simultaneous drilling and logging of well boreholes.

This is .a division of the copendingScherbatskoy applicat'ionSerial No. 173,165, filed July 11, 1950.

Heretofore, in the conventional practice of well bore- 'hole logging, a suitable sourceof electric current has been the current therefrom .is applied through a suitably insulated conductor cable extending into the borehole to sensing apparatus therein to provide an electric current or signal in the conductor cable representative of the value of certain physical or electrical characteristics or .quantities desired to be measured within the well borehole. The provision .and maintenance vof such insulated conductors in a drilling well, together with the drill pipe, 'in :such a manner that drilling andelectrical logging operations can 'be carried on simultaneously, have been found to be impracticable. Therefore, the usual well logging practice has been to interrupt the well drilling operation at suitable intervals to permit the removal of the drill pipe from the borehole, .and the running into the bore- 'hole of the before-mentioned conventional logging apparatus suspended from an insulated conductor cable while the drill pipe is absent from the borehole. The provision and maintenance of an insulated conductor cable forwell borehole logging under any circumstances, whether 'or not present in the borehole together with "the drill pipe, are diflicult and costly.

The before-mentioned conventional intermittent logging practice has the disadvantage -that the precise control at all times of the depth of drilling with respect to certain formations is difficult. Consequently, in many cases the desired shale body or the candidate productive formation may have been drilled through or passed by between the logging operation intervals, thereby possibly necessitating subsequent time-consuming and expensive corrective measures before the next intended step in the process. of -con- "is progressive with time and, if permitted to continue for an appreciable length of time a'fter drillin eforethe logging measurements are taken, may result in suliicient contamination of the formations to .cause possibleyconfusion in the correct interpretation of some 'of the electrical characteristics of the formation thus measured.

. The before-described 'dilficulties are largely overcome by the present invention, which does not require an insulated conductor cable in the borehole at anytime, but provides for the instant-transfer of the results of the electrical logging operations or 'other s'imilargmeasure- 'ments made *within the borehole, to the eartlis surface I located at the earths surface outside of theborehole, and

withoutemploying the usual interconnecting insulating conductors, and .thisis accomplished while the drill pipe is within :the well borehole and continuously during and simultaneously with drilling operations. The system of the present inventionpermits the electrical logging or .other measuring apparatus to be contained within .the lower end .of the drill stem, preferably within the drill collar, and the process Inf-making the actual logging measurements to be carried on there simultaneously with the drilling of the borehole, whereby the electrical loggingmeasurements can be made of the freshly penetrated formation before excessive invasion of drilling fiuidinto the formation takes place.v

Another advantage of the present invention resides in the simultaneous .drilling and .logging .of the formation which it makes possible, by the continuous transmission of-signals through the drilling fluid in the drill stem or .in the annulus within the borehole surrounding the drill :stem toreceiving apparatus at the earthssurface, whereby a log representative .of the formations being drilled through is continuously available.

Accordingly, an object of this .inventionis to provide a method and apparatus for logging earth boreholes or the measurement of electrical quantities within well boreholes, in which the .necessityfor using an insulated conductor extending into the borehole to the place of measurementis avoided. Y 1

.Anothenobjectof this inventionis to provide a logging system which permits the conducting of loggingmeasure- .ment operationsand drilling operations simultaneously.

Another-object of this invention is to provide a new method o'fand apparatus for the transmission of measure-' equally applicable to the exploration of boreholes simultaneously with the drilling operation and to the exp1ora tion of boreholes in which the drilling either has been completed or has been suspended.

The objects of this invention are attained, in brief, by utilizing logging methods for detecting and measuring the variations in the earth characteristics .or in measuring values of other physical quantities at a point in the borehole adjacent the drill bit while drilling ,is in progress, making a continuous measurement of such characteristics or values by suitable means located in the drill stem adjacent the drill bit, converting such measurements into pressure wave impulses inthe drilling fluid, by means of explosive means located at or adjacent .the lower end of the drill stem, whereby such impulses travel upward through the drilling fluid in the drill stem or in the annular space surrounding the drill stem, to the ,earths surface, and receiving and translating these impulses at the earths surface into suitable electrical signals indicative of the measurements made the well, for recording graph- ..ically on a chart in correlation with the depth of the corresponding drilling operations or point of measurement.

.These and other objects, advantages, and features of tnoveltywill be evident hereinafter in the following more detailed description of the invention.

In the.drawings,vwhich illustrate preferred embodiments and modes of operation of the invention, and in which like reference characters designate the same or similar parts throughout the several views:

Figure lris a vertical sectional elevational view illustrative of 'a typical well borehole, showing the general arrangement of the apparatus of the invention, partially in elevation and partially diagrammatically;

"Figure 2 is a schematic wiring diagram of a transducer suitable for use in the apparatus of Figure 1 'sulated from the drill collar and from one another. before-mentioned insulating coating, covering, or sleeve Figure 3 is an enlarged cross-sectional view taken on line 3--3 of Figure l; and

Figure 4 is a fragmentary vertical sectional view of the apparatus taken on line 4-4 of Figure 3.

Referring first primarily to Figure l, a longitudinal section of a typical well borehole is shown, by way of example, having a lower uncased portion and an upper portion in which a surface string of casing 11 has been set in accordance with conventional practice. Within the borehole is shown a substantially conventional rotary drill string comprising a drill bit 12, a special drillcollar 13, and a conventional drill stem or drill pipe 14. The drill string is rotated by means of a conventional rotary table and is supported by conventional well derrick apparatus (not shown).

Provision is made for circulating drilling fiuidby introducing it under pressure from drilling fluid circulation pumps (not shown) into the top of the drill pipe in conventional manner, and thence down through the passages in the drill collar to be discharged through apertures 22 in the drill bit 12 into the bottom of the hole. The drilling fluid thus introduced into the drill stern and dislar, and, therefore, when the battery circuit from the charged from the drill bit circulates in return from the bottom of the borehole upward through the annular space between the borehole and the drill stem to the top of the borehole, from which it is finally discharged through a side outlet connection 24 leading from the surface casing 11 for return to a drilling fluid sump, from which it may be withdrawn and recirculated by the'circulating pumps 'as just described. 7

The drill collar 13, which is attached to the lower end of the drill pipe 14, as before mentioned, comprises a lower logging measurement and signal-generating instrumentcontaining portion 13a and an upper signal-transmitting portion 13b. The lower logging measurement and signalgenerating portion 13a of the drill collar is provided with an outside electrical insulating coating, covering, or sleeve 17 which serves to electrically insulate that portion of the drill collar from the surrounding drilling fluid and also serves to support a pair of longitudinally spaced-apart annular electrode rings 25 and 26, which are thereby in- The 17 may be composed of suitable insulating material, such as rubber, neoprene, Bakelite, or other suitable insulating, abrasion-resistant materials. The drill bit'12 preferably is insulated electrically from the drill collar 13 by suitable means (not shown) such as a suitable threaded insulating bushing in the threaded joint between the lower end of the drill collar and the drill bit.

The logging measurement and signal-generating apparatus, which are contained within suitable fluid-tight enclosures within the lower portion 13a of the drill collar 13, are illustrated diagrammatically within the dotted rectangular enclosure 30. The receiving and recording portion of the apparatus located at the earths surface outside of the well borehole is illustrated within the dotted 'means of an insulated conductor 37 to a suitable terminal 38 electrically connected at any suitable point to the metal body of the drill collar 13. An electric switch (not shown) operated by the flow of the drillingfluid may be employed to keep the battery disconnected from the electrode 20 whenever the mud pumps are shut down for any reason, such as during the time when the drilling string is being lowered or withdrawn from the hole or when drilling operations are interrupted for any other reason.

The ground terminal 38in the drill collar is, as before mentioned, grounded to the metal body of the drill colbattery 35 through the constant current element 39 to the terminal 38 and the electrode 25 is completed by closure of the fluid-actuated switch (not shown), an electric current of substantially constant value is permitted to flow from the positive terminal of the battery 35 through the insulated conductor 36 to the annular electrode 25, and thence through the surrounding drilling fluid out into the adjacent earth formation and return from the adjacent earth formation through the surrounding drilling fluid in the borehole to the metal portion of the drill collar, and from there through the insulated conductor 37 and constant current element 39 back to the negative terminal of the battery 35. As a result of this current flow, a potential difference or voltage is established between the annular electrodes 25 and 26, the magnitude of which is representative of the before-mentioned resistivity of the adjacent formation surrounding the borehole.

The voltage thus picked up between the electrodes 25 and 26, which is representative of the resistivity of the adjacent formation surrounding the borehole, is applied through conductors 42 and 43 and filter 44 to the input of a D.C. amplifier 45. The filter 44 may be of substantially conventional design, adapted to pass only very low frequencies and to exclude noise frequencies above the range of frequencies involved in the desired logging measurements. The resultant D.-C. output from the amplifier 45 is applied through conductors 46 and 47 to the input of a multivibrator 48. The multivibrator 48 may be of any well known type capable of producing output pulsations at a frequency which is proportional to or bears a predetermined functional relationship to the D.-C. input control potential as applied thereto, for example, in the present application from the D.-C. amplifier 45 through the conductors 46 and 47, as before mentioned.

The pulsating output signal from the multivibrator 48 is conducted through leads 49 and 50 to the input of a scaling circuit 55, which acts as a demultiplier. The sealing circuit may be of any well known type capable of transforming a given input signal frequency to a lower output signal frequency which bears a predetermined ratio to the said input frequency. Thus, by the action of the scaling circuit or demultiplier 55, the input pulsations received by it through conductors 49 and 50 are reduced in frequency therein, and the resultant, relatively low frequency output therefrom applied through conductors 56 and 57 to amplifier 59.

Thus the scaling circuit or demultiplier 55 receives at its input terminals, through conductors 49 and 50, a succession of pulses, the frequency of said pulses representing at any given instant the resistivity of the formationsurrounding the borehole adjacent the electrodes 25 and 26. The scaling circuit produces, however, at its output terminals pulses of a lower frequency. When the resistivity of the formation adjacent the electrodes 25 and 26 changes, thereby causing a corresponding change in the potential applied from the amplifier 45 to the multivibrator 48 and thence from the multivibrator 48 to the scaling circuit, the frequency of the output pulses from the scaling circuit correspondingly changes in such a manner that the ratio of the input frequency to the output frequency has a constant and relatively large value. A succession of pulses is therefore obtained across the output terminals of the scaling circuit'55 having a much larger time spacing than the drilled.

These relatively low frequency output pulses applied from the scaling circuit to the amplifier 59 are there amplified and applied through conductors 60 and 61 to the windings of the electromagnet 63 of a ratchet-type sequential switch 64. The ratchet-type switch 64 includes an armalure 65 located adjacent to and adapted to be actuated ment-containing portion 13a of the drill collar.

by the electromagnet 63, and has attached thereto a pawl 67 adapted to make one-way ratcheting engagement with the ratchet teeth formed on the periphery of a ratchet wheel 68 in such manner that each time the armature 65 is actuated by the electromagnet 63, the ratchet wheel 68 is rotated step-wise in counterclockwise direction through a predetermined angle.

The ratchet wheel 68 of switch 64 carries a switch arm 69 for rotation therewith step-wise into sequential contact with a plurality of electrical contact points, as shown at 70. Each of the contact points 70 of the ratchet switch is connected through a conductor within the drill collar, such as diagrammaticallyshown at 90, to an ignition'filament 88 and thence to ground 92, with completion of the electrical circuit through the ground connection, the drill collar body, to battery 94, which is in turn connected through conductor 96 to the before-mentioned ratchet switch arm 69. Each of the ignition filaments S8 is located in an explosive signal unit contained within the upper signal-transmitting port-ion 13b of the drill collar 13, as will be hereinafter more fully described.

As before mentioned, each of the contact points 70 of the ratchet switch 64 is connected by a separate insulated conductor lead, as shown at 90 in Figures fl, 3, and 4, to a separate one of the plurality of ignition filaments of the powder chambers or explosive cartridges contained in the signal units, as shown at 100' in Figures 3 and 4. Each of the explosive signal units comprises a replaceable, externally threaded body or plug 102 threaded into suitable, internally threaded, laterally outwardly facing sockets, as shown at 103, formed in the outside surface of the upper signal-transmitting portion 13b of the drill collar 13. Each of the signal unit bodies 192 is formed with a coaxial inner chamber 105 for containing a suitable cartridge or quantity of powder. The before-mentioned ignition filament 88 extends through the chamber or cartridge from a suitable gasand fluid-tight conductor inlet connection 107 at the rear end of the chamber to a frangible diaphragm 104 located at the forward end of the chamber. Each of the ignition filaments '88 is thus grounded to the drill collar body (as schematically illustrated at v92 in Figure 1) at its outer end through the frangible metal diaphragm 104. The diaphragm 104 serves as a fluid-tight closure for the outer ends of chambers 105, and each metal diaphragm is held in place by means of a threaded annular nut 111 and an intermediate annular gasket 108.

'The several conductors 94) which make electrical interconnection between the individual switch contact points 70 and the several corresponding ignition filaments '88 contained in the before-mentioned signal units, extend through suitably positioned, longitudinal ducts in the drill collar body, as shown at 110 in Figures 3 and 4, and interconnect the upper signalling portion 13b and the lower instru- The conductor inlet connect-ion 107 extends rearwardly from the rear end of the signal unit body into the duct 11.0, where electrical connection is made with the conductor 90. For the purpose of simplification, only three of the conductors 90 are shown in each of the ducts 110, although the actual number of such conductors would be considerably greater, the total number being equal to the number of explosive signal units contained in the upper signalling portion of the drill collar 13b, and alsoequal to the total numberof switch contact points 79 of the ratchet switch 64.

The surface apparatus for receiving and recording the signals transmitted from the hereinbefore-described subsurface signal transmitting apparatus is, as hereinbefore mentioned, shown diagrammatically within the dotted rectangular enclosure 31. This receiving apparatus includes a microphone M which is connected acoustically through a short length of pipe 112 to a side opening in the surface casing 11 at a point below the fluid level therein and preferably, although not necessarily, at a sufii'cient "distance under the surfaceof the earth to be relatively free of engine and exhaust noises. The output of the microphone M is connected through conductors 1'14 and 11 5 to a filter 116, and thence through conductors 117 and 118 to the input of a D. C. amplifier 119. The output from the amplifier 119 is connected through conductors 121 and 122 to the input of a transducer 123 (see Figure 2). The output from the transducer 123 is connected through conductors 125 and 126 to the input terminals of a suitable meter or recorder such as that illustrated at 127. The recorder 127 is provided with a strip chart 128 adapted to be driven by means of a suitable depth meter 129 such that the motion of the strip chart 128 relative to the record ing pen 130 is maintained in predetermined correlation with the depth of the electrodes 25 and 26 within the borehole 10. Suitable apparatus which may be adapted to perform the service of the depth meter 129 is shown in Figures 2, 3, and 4 of the copendin'g Alps application, Serial No. 90,503, filed April 29, 1949.

Referring now to Figure 2, a structure is schematically illustrated there of a transducer suitable for employment at 123 in Figure 1. This transducer is adapted to receive input pulsations at various time intervals and at translate such time intervals into corresponding output voltages which may be measured by suitable apparatus, such as the recorder 127. The transducer 123 is thus adapted to receive electrical impulses across its input terminals 121 and 122 and produce across the output terminals 125 and 126 corresponding voltages representing the duration of the time intervals between the successive input impulses. The transducer network comprises a capacitance '131 connected in series with a constant current element 132 and a current supply battery 133, said capacitance, constant current element, and battery forming a closed circuit. The capacitance 131 is in turn connected to another closed circuit consisting of conductor 135, which makes connection with the .anode of a triode V, the cathode of which is connected through resistor 137 to ground and thence in return to the capacitance 131. Input conductor '121 is connected through a negative bias battery '138 to the grid of the triode V, and the other input conductor 122 is connected directly to the cathode of the triode V. A grid bias resistor 124 is connected between the cathode and the positive side of the bias battery 138.

The output conductors 125 and 126 are connected 'to opposite ends of the cathode resistor 137. The voltage of battery 138 is such as normally .to maintain negative bias on the grid of the triode V suflicient to maintain the triode non-conductive for all extraneous noise pulsations which may reach the input terminals 121 and 122. Consequently, the triode V is normally non-conductive and becomes conductive only at the times when the relatively high signal voltage impulses are applied to the terminals 121 and 122.

The operation of the apparatus is as follows:

Current flows from the battery 35 through constant current element 39 and conductor 3'6 to electrode .25 and out through the surrounding drilling fluid and adiacent formations and return through the surrounding drilling fluid to the exposed uninsulated portion of the surface of the drill collar, and thence through conductor 37 to the battery 35. The current thus applied .to the formations surrounding the borehole will be of substantially constant value, and a potential field will thereby be set up in the surrounding formations, in the manner well known in the electrical logging art, which is a function of the resistivity of such surrounding formation-s. A portion of this potential field or voltage, representative of the resistivity of the surrounding formations adjacent the electrodes 25 and 26, is picked up by these electrodes and is applied through conductors 42 and 43 and filter 44 to the input of amplifier 45. Any other suit able arrangement of electrodes, well known in the electrical logging art, may be employed, from which a potential difference can be obtained, for application to the input to amplifier 45, and which is representative of the "value.

Lwhich the charging step is permitted to continue.

.resistance, resistivity, or other desired electrical characteristic of a portion of the adjacent formations. Likewise,

a potential difference representative of any other physi cal quantity desired to be measured within the borehole may be applied to the input of amplifier 45. The resultant D.-C. output from the amplifier 45, which is proportional to or bears a predetermined. functional relation- .Shipto the input potential such as the potential picked up ,betweenelectrodes 25 and 26, is a plied through conductors 46 and 47m the input of multivibrator 48. The resultant output from the multivibrator 48, which is a pulsating or alternating potential having a frequency which bears a predetermined relationship to the input 'D.-'C. potential, is applied through conductors 49 and 50 to the input of the scaling circuit 55. The scalingcircuit 55'produces a pulsating or alternating output potential which has a frequency very much lower than the input frequency, but which bears a given constant relationship thereto. The thus-produced relatively low frequency pulsating or alternating current output signal from the scalingcircuit is appliedthrough conductors 56 and 57 to the amplifier 59, and from the output of the amplifier 59 to the windings of the electromagnet '63 for actuation of the ratchet switch 64.

Each time .the scaling circuit produces an output pulsation, the armature 65 of the ratchet switch 64-is actuwhich travels upward through the drilling, fluid in the borehole to theearths surface, where it actuates the microphone/M and the receiving circuit attached there to, as hereinafter described.

Each such impulse transmitted to the earths surface through the drilling fluid, when detected by the microphone M, produces an electrical impulse in conductors 114 and 115 which, after passing through filter 116, is applied through conductors 117 and 118 to the input of the amplifier 119. The resultant amplified impulses from amplifier 119 are applied through conductors 121 and 122 to the grid and cathode of the triode V of the transducer 123. The voltage impulses thus applied to the input of the transducer are of a magnitude and a polarity suflicient to overcome the negative bias provided by battery 138 sufficiently to cause the tube to become conductive and to permit a substantially complete discharge of the capacitance 131 through the resistor 137. Immediately thereafter, the conductivity of the triode V ceases, and the capacitance 131 commences to be again charged by the battery 133 through the constant current device 132. By reason of the action of the constant current device 132, the charging current, and consequently the rate of the charging of the condenser, is maintained at a relatively small, substantially constant Consequently, the voltage across the capacitance 131 increases substantially linearly with time during This linear increase in potential continues until the instant at which the next succeeding signal input pulse appears across the input terminals 121 and 122 of the trans- ,ducer, then, at that instant, the triode V again becomes conductive, and the capacitance 131 (the voltage therein .having reached a value proportional to the time interval between the two succeeding pulsations) discharges rapidly vthrough the resistor 37 and thereby produces across the resistor 137 and between the output connections 125 and 126 a peak voltage having an amplitude proportional -,to or indicative of the time interval between the preceding rimpulses, Immediately afterwards, the conductivity of? the triode V again ceases, and the charging of the capacik tance 131 by the battery 133 is again resumed, and the only, and that the invention is not to be limited thereby,

voltage across the capacitance 131 begins again to increase substantially linearly with time from the instant of the reestablishment of the non-conductivity of the tube until. the instant when the next succeeding impulse appears across the input terminals 121 and 12,2 of the transducers, At this next instant, after the condenser voltage has reached a value proportional to the time elapsing from the next preceding input impulse, the; tube V again becomes instantly conductive, thereby. causing the capacitance 131 again to discharge through the resistor 137. These cycles of'operation are repeated in succession so long as the apparatus is in operation, the resultant peak voltage impulses being thus applied to the recorder 127 through the conductors and 126, having values proportional to or beating a predetermined functional relationship to the time intervals between such pulsations, which are, in turn, as hereinbefore expalined,

indicative of the resistance or resistivity of the formations surrounding the borehole adjacent the electrodes 25 and The potentialsapplied through conductors 125 and 126 to the recorder 127 cause the pen to move laterally across the chart 128, as, the chart 128 is moved longitudinally by means of the depth meter 1:29 in correlation'with thedepth'oi the electrodes 25. and 26 within the borehole, to produce the curve or graph as shown at 140. The curve or graph thus plotted on thechart 128 constitutes the electrical log of the well borehole 10.

The method of the present invention hereinbefore mation resistivity over the time intervals between pulses is produced by the recorder. Only one kind of explosive signal is required, and the surface recorder does not .need to diiterentiate between strong and weak signals.

I it is to be understood that the foregoing is illustrative but includes all modifications thereof within the scope of the invention as described in the appended claims.

What is claimed is:

1. A method of logging boreholes comprising: measuring at given depths within a borehole values of a characteristic of the formation adjacent the borehole at such depths; producing a signal therein in response to the measurement thus obtained and having a form respresentative of the said values of said characteristic; and igniting separate explosive charges within said bore hole in the vicinity of said given depths in response to said signal and having between consecutive ignitions thereof time intervals each bearing a predetermined functional relationship to said signal, whereby corresponding compressional waves are generated within said borehole at corresponding time intervals which are indicative of the said values measured.

2. A method of logging boreholes comprising: measuring at given depths within a borehole values of a characteristic of the formation adjacent the borehole at such depths; producing a signal therein in response to the measurement thus obtained and having a magnitude representative of the said values of said characteristic; and igniting separate explosive charges within said borehole in the i of the said values measured.

3. A method of logging boreholes comprising: measuring at given depths within a borehole values of a characteristic of the formation adjacent the borehole at such depths;

producing a signal therein in response to the measurement thus obtained and having a freuency representative of the said values of said characteristic; and igniting separate explosive charges within said borehole in the vicinity of said given depths in response to said signal, at a frequency having a predetermined functional relationship to the said frequency of said signal, whereby corresponding compressional waves are generated in said bore hole at frequencies which are indicative of the said values measured.

4. A method of logging boreholes comprising: measuring at a given depth within a borehole the value of a physical quantity to be measured therein; and firing a pair of separate explosive charges within said bore hole in the vicinity of said measurement, at a time interval therebetween which is indicative of the said value measured.

5. A method of logging boreholes comprising: measuring at a given depth within a borehole the values of a physical quantity to be measured therein; producing, in response to said measurement, a signal having a frequency indicative of said values measured; and firing separate explosive charges within said borehole in the vicinity of said measurement, at time intervals therebetween, each of which time intervals bears a predetermined functional relationship to said frequency of said signal.

6. A method of logging boreholes comprising: measuring at given depths within a borehole the value of a physical quantity to be measured therein; producing, in response to said measurement, a signal having a form indicative of said value measured; and firing separate explosive charges within said borehole in the viicnity of said measurement, at time intervals therebetween, each of which time intervals bears a predetermined functional relationship to said form of said signal.

7. A method of logging boreholes comprising: measuring at given depths within a borehole the values of a char acteristic of the formation adjacent the borehole at such depths; producing a signal therein in response to the measurement thus obtained and having a form representative of the said values of said characteristic; igniting separate explosive charges within said borehole in the vicinity of said given depths, in response to said signal and having, between the consecutive firings of such charges, time intervals each indicative of the form of said signal, whereby corresponding compressional waves are generated in said borehole at intervals therebetween, each of which time intervals is indicative of the said values measured; and receiving and converting said compressional waves adjacent the top of the borehole into received signals indicative of said time intervals between said compressional waves.

8. A method according to claim 7, and measuring said received signals in correlation with the said given depths.

9. Apparatus for logging boreholes comprising: measuring means for measuring at a given depth within a borehole the value of a physical quantity; and means operative in response to said measuring means for firing a pair of separate explosive charges within said bore hole in the vicinity of said measurement means at a time interval therebetween which is indicative of the said value measured.

10. Apparatus for logging a borehole during drilling, in which a fluid stream is circulated through a drill stem into and out of the borehole, comprising: means adjacent the lower end of the drill stern for producing a logging signal therein having values representative of values of a physical quantity measured adjacent said lower end of said drill stem in said borehole; means adjacent the lower end of said drill stem for containing a plurality of explosive signal units adapted to be discharged into said fluid stream; means responsive to said logging signal for firing separate ones of said explosive signal units at time intervals therebetween, which time intervals are each indicative of the values of said logging signal; means adjacent the top of said borehole, in communication with said fluid stream, for receiving compressional waves generated in said fluid stream by said explosive signal units; and means to generate, in response to said received compressional i9 waves, a received signal bearing a predetermined relationship to the time intervals between said received compressional waves and thus indicative of the values of said logging signal.

1]. Apparatus in accordance with claim 10, and means to plot said received signal in correlation with the depth of said drill stem in said borehole.

12. Apparatus in accordance with claim 9 in which the said measuring means comprises means for measuring the resistivity of adjacent formations surrounding the borehole.

13. Apparatus in accordance with claim 9 in which the said measuring means comprises means for measuring the resistance of adjacent formations surrounding the borehole.

14. Apparatus in accordance with claim 10 in which the physical quantity measured comprises the resistivity of adjacent formations surrounding the borehole.

15. Apparatus for logging a borehole during drilling in which a fluid stream is circulated through a drill stem into and out of the borehole, comprising: means adjacent the lower end of said drill stem for measuring a physical quantity; means adjacent the lower end of said drill stem for producing a pulsating electric logging signal therein having a pulsation frequency responsive to and bearing a predetermined functional relationship to said measured physical quantity; means adjacent said lower end of said drill stem for containing a plurality of explosive signal units adapted to be discharged into said fluid stream when fired; electric means for firing said explosive signal units; means responsive to said logging signal for producing a second pulsating electric signal having a pulsation frequency bearing a predetermined relationship to the pulsation frequency of said logging signal; and switching means electrically connected to said electric means for electrically firing separate ones of said explosive signal units consecutively at time intervals, said switching means being responsive to and actuated at a rate corresponding to the said pulsation frequency of said second pulsating electric signal, whereby the time intervals between the firing of said explosive units bear a predetermined relationship to the pulsation frequency of said second electric signal.

16. Apparatus according to claim 15 in which the said means responsive to said logging signal includes means for producing the second pulsating electric signal having a pulsation frequency which is a predetermined multiple of the pulsation frequency of said logging signal.

17. Apparatus according to claim 16, and means to be located adjacent the top of said borehole in communication with said fluid stream for receiving compressional waves generated in said fluid stream by said explosive signal units when fired, and to generate, in response to said received compressional Waves, a received signal bearing a predetermined relationship to the time intervals between said received compressional waves, said received signal thus being indicative of the values of said logging signal.

18. Apparatus for logging a borehole comprising: measuring means for measurements at given depths within a borehole of the values of a physical quantity; means for producing, within such borehole, in response to said measurements, a signal having a form indicative of the values measured by said measuring means; and means for firing separate explosive charges within said borehole in the vicinity of said measurements at time intervals between said firings, each of which time intervals bears a predetermined functional relationship to said form of said signal.

References Cited in the file of this patent UNITED STATES PATENTS 2,262,419 Athy et a1 Nov. 11, 1941 2,425,869 Dillon Aug. 19, 1947 2,507,351 Scherbatskoy May 9, 1950 2,677,790 Arps May 4, 1954 

1. A METHOD OF LOGGING BOREHOLES COMPRISING: MEASURING AT GIVEN DEPTH WITHIN A BOREHOLE VALUES OF A CHARACTERISTIC OF THE FORMATION ADJACENT THE BOREHOLE AT SUCH DEPTHS; PRODUCING A SIGNAL THEREIN IN RESPONSE TO THE MEASUREMENT THUS OBTAINED AND HAVING A FORM REPRESENTATIVES OF THE SAID VALUES OF SAID CHARACTERISTIC; AND IGNITING SEPARATE EXPLOSIVE CHARGES WITHIN SAID BORE HOLE IN THE VICINITY 